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<a name="Macros-for-Initialization"></a>
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<a name="Macros-Controlling-Initialization-Routines"></a>
<h4 class="subsection">18.20.6 Macros Controlling Initialization Routines</h4>

<p>Here are the macros that control how the compiler handles initialization
and termination functions:
</p>
<dl>
<dt><a name="index-INIT_005fSECTION_005fASM_005fOP-1"></a>Macro: <strong>INIT_SECTION_ASM_OP</strong></dt>
<dd><p>If defined, a C string constant, including spacing, for the assembler
operation to identify the following data as initialization code.  If not
defined, GCC will assume such a section does not exist.  When you are
using special sections for initialization and termination functions, this
macro also controls how <samp>crtstuff.c</samp> and <samp>libgcc2.c</samp> arrange to
run the initialization functions.
</p></dd></dl>

<dl>
<dt><a name="index-HAS_005fINIT_005fSECTION"></a>Macro: <strong>HAS_INIT_SECTION</strong></dt>
<dd><p>If defined, <code>main</code> will not call <code>__main</code> as described above.
This macro should be defined for systems that control start-up code
on a symbol-by-symbol basis, such as OSF/1, and should not
be defined explicitly for systems that support <code>INIT_SECTION_ASM_OP</code>.
</p></dd></dl>

<dl>
<dt><a name="index-LD_005fINIT_005fSWITCH"></a>Macro: <strong>LD_INIT_SWITCH</strong></dt>
<dd><p>If defined, a C string constant for a switch that tells the linker that
the following symbol is an initialization routine.
</p></dd></dl>

<dl>
<dt><a name="index-LD_005fFINI_005fSWITCH"></a>Macro: <strong>LD_FINI_SWITCH</strong></dt>
<dd><p>If defined, a C string constant for a switch that tells the linker that
the following symbol is a finalization routine.
</p></dd></dl>

<dl>
<dt><a name="index-COLLECT_005fSHARED_005fINIT_005fFUNC"></a>Macro: <strong>COLLECT_SHARED_INIT_FUNC</strong> <em>(<var>stream</var>, <var>func</var>)</em></dt>
<dd><p>If defined, a C statement that will write a function that can be
automatically called when a shared library is loaded.  The function
should call <var>func</var>, which takes no arguments.  If not defined, and
the object format requires an explicit initialization function, then a
function called <code>_GLOBAL__DI</code> will be generated.
</p>
<p>This function and the following one are used by collect2 when linking a
shared library that needs constructors or destructors, or has DWARF2
exception tables embedded in the code.
</p></dd></dl>

<dl>
<dt><a name="index-COLLECT_005fSHARED_005fFINI_005fFUNC"></a>Macro: <strong>COLLECT_SHARED_FINI_FUNC</strong> <em>(<var>stream</var>, <var>func</var>)</em></dt>
<dd><p>If defined, a C statement that will write a function that can be
automatically called when a shared library is unloaded.  The function
should call <var>func</var>, which takes no arguments.  If not defined, and
the object format requires an explicit finalization function, then a
function called <code>_GLOBAL__DD</code> will be generated.
</p></dd></dl>

<dl>
<dt><a name="index-INVOKE_005f_005fmain"></a>Macro: <strong>INVOKE__main</strong></dt>
<dd><p>If defined, <code>main</code> will call <code>__main</code> despite the presence of
<code>INIT_SECTION_ASM_OP</code>.  This macro should be defined for systems
where the init section is not actually run automatically, but is still
useful for collecting the lists of constructors and destructors.
</p></dd></dl>

<dl>
<dt><a name="index-SUPPORTS_005fINIT_005fPRIORITY"></a>Macro: <strong>SUPPORTS_INIT_PRIORITY</strong></dt>
<dd><p>If nonzero, the C++ <code>init_priority</code> attribute is supported and the
compiler should emit instructions to control the order of initialization
of objects.  If zero, the compiler will issue an error message upon
encountering an <code>init_priority</code> attribute.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fHAVE_005fCTORS_005fDTORS"></a>Target Hook: <em>bool</em> <strong>TARGET_HAVE_CTORS_DTORS</strong></dt>
<dd><p>This value is true if the target supports some &ldquo;native&rdquo; method of
collecting constructors and destructors to be run at startup and exit.
It is false if we must use <code>collect2</code>.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fDTORS_005fFROM_005fCXA_005fATEXIT"></a>Target Hook: <em>bool</em> <strong>TARGET_DTORS_FROM_CXA_ATEXIT</strong></dt>
<dd><p>This value is true if the target wants destructors to be queued to be
run from __cxa_atexit.  If this is the case then, for each priority level,
a new constructor will be entered that registers the destructors for that
level with __cxa_atexit (and there will be no destructors emitted).
It is false the method implied by <code>have_ctors_dtors</code> is used.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fASM_005fCONSTRUCTOR"></a>Target Hook: <em>void</em> <strong>TARGET_ASM_CONSTRUCTOR</strong> <em>(rtx <var>symbol</var>, int <var>priority</var>)</em></dt>
<dd><p>If defined, a function that outputs assembler code to arrange to call
the function referenced by <var>symbol</var> at initialization time.
</p>
<p>Assume that <var>symbol</var> is a <code>SYMBOL_REF</code> for a function taking
no arguments and with no return value.  If the target supports initialization
priorities, <var>priority</var> is a value between 0 and <code>MAX_INIT_PRIORITY</code>;
otherwise it must be <code>DEFAULT_INIT_PRIORITY</code>.
</p>
<p>If this macro is not defined by the target, a suitable default will
be chosen if (1) the target supports arbitrary section names, (2) the
target defines <code>CTORS_SECTION_ASM_OP</code>, or (3) <code>USE_COLLECT2</code>
is not defined.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fASM_005fDESTRUCTOR"></a>Target Hook: <em>void</em> <strong>TARGET_ASM_DESTRUCTOR</strong> <em>(rtx <var>symbol</var>, int <var>priority</var>)</em></dt>
<dd><p>This is like <code>TARGET_ASM_CONSTRUCTOR</code> but used for termination
functions rather than initialization functions.
</p></dd></dl>

<p>If <code>TARGET_HAVE_CTORS_DTORS</code> is true, the initialization routine
generated for the generated object file will have static linkage.
</p>
<p>If your system uses <code>collect2</code> as the means of processing
constructors, then that program normally uses <code>nm</code> to scan
an object file for constructor functions to be called.
</p>
<p>On certain kinds of systems, you can define this macro to make
<code>collect2</code> work faster (and, in some cases, make it work at all):
</p>
<dl>
<dt><a name="index-OBJECT_005fFORMAT_005fCOFF"></a>Macro: <strong>OBJECT_FORMAT_COFF</strong></dt>
<dd><p>Define this macro if the system uses COFF (Common Object File Format)
object files, so that <code>collect2</code> can assume this format and scan
object files directly for dynamic constructor/destructor functions.
</p>
<p>This macro is effective only in a native compiler; <code>collect2</code> as
part of a cross compiler always uses <code>nm</code> for the target machine.
</p></dd></dl>

<dl>
<dt><a name="index-REAL_005fNM_005fFILE_005fNAME"></a>Macro: <strong>REAL_NM_FILE_NAME</strong></dt>
<dd><p>Define this macro as a C string constant containing the file name to use
to execute <code>nm</code>.  The default is to search the path normally for
<code>nm</code>.
</p></dd></dl>

<dl>
<dt><a name="index-NM_005fFLAGS"></a>Macro: <strong>NM_FLAGS</strong></dt>
<dd><p><code>collect2</code> calls <code>nm</code> to scan object files for static
constructors and destructors and LTO info.  By default, <samp>-n</samp> is
passed.  Define <code>NM_FLAGS</code> to a C string constant if other options
are needed to get the same output format as GNU <code>nm -n</code>
produces.
</p></dd></dl>

<p>If your system supports shared libraries and has a program to list the
dynamic dependencies of a given library or executable, you can define
these macros to enable support for running initialization and
termination functions in shared libraries:
</p>
<dl>
<dt><a name="index-LDD_005fSUFFIX"></a>Macro: <strong>LDD_SUFFIX</strong></dt>
<dd><p>Define this macro to a C string constant containing the name of the program
which lists dynamic dependencies, like <code>ldd</code> under SunOS 4.
</p></dd></dl>

<dl>
<dt><a name="index-PARSE_005fLDD_005fOUTPUT"></a>Macro: <strong>PARSE_LDD_OUTPUT</strong> <em>(<var>ptr</var>)</em></dt>
<dd><p>Define this macro to be C code that extracts filenames from the output
of the program denoted by <code>LDD_SUFFIX</code>.  <var>ptr</var> is a variable
of type <code>char *</code> that points to the beginning of a line of output
from <code>LDD_SUFFIX</code>.  If the line lists a dynamic dependency, the
code must advance <var>ptr</var> to the beginning of the filename on that
line.  Otherwise, it must set <var>ptr</var> to <code>NULL</code>.
</p></dd></dl>

<dl>
<dt><a name="index-SHLIB_005fSUFFIX"></a>Macro: <strong>SHLIB_SUFFIX</strong></dt>
<dd><p>Define this macro to a C string constant containing the default shared
library extension of the target (e.g., &lsquo;<samp>&quot;.so&quot;</samp>&rsquo;).  <code>collect2</code>
strips version information after this suffix when generating global
constructor and destructor names.  This define is only needed on targets
that use <code>collect2</code> to process constructors and destructors.
</p></dd></dl>

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